mysql - 获取 SQL UNION 中的字段总和

Question

我将如何在这里获得共同领域的总和？

SELECT first_user_id, mutual
    FROM userprofile_usercontact
UNION ALL
SELECT second_user_id, mutual
    FROM userprofile_usercontact
GROUP BY first_user_id

换句话说，我想获得SUM(mutual)（first_user_id + second_user_id），其中mutual = 1。

Answer 1

SELECT u.user_id, SUM(u.mutual)
  FROM (SELECT first_user_id AS user_id, mutual
          FROM userprofile_usercontact
        UNION ALL
        SELECT second_user_id AS user_id, mutual
          FROM userprofile_usercontact
       ) AS u
 GROUP BY user_id;

评论中的问题是：

我将如何摆脱相互 = 0 的条目？

像往常一样使用过滤条件：

SELECT u.user_id, SUM(u.mutual)
  FROM (SELECT first_user_id AS user_id, mutual
          FROM userprofile_usercontact
        UNION ALL
        SELECT second_user_id AS user_id, mutual
          FROM userprofile_usercontact
       ) AS u
 WHERE u.mutual != 0   -- or u.mutual <> 0
 GROUP BY user_id;

优化器很可能会将条件下推到 UNION 查询中，因此它会像编写查询一样执行：

SELECT u.user_id, SUM(u.mutual)
  FROM (SELECT first_user_id AS user_id, mutual
          FROM userprofile_usercontact
         WHERE mutual != 0
        UNION ALL
        SELECT second_user_id AS user_id, mutual
          FROM userprofile_usercontact
         WHERE mutual != 0
       ) AS u
 GROUP BY user_id;

最终结果当然是一样的。

Answer 2

SELECT SUM(mutual) sm, user_id FROM
(SELECT first_user_id user_id, mutual
    FROM userprofile_usercontact
    UNION ALL
    SELECT second_user_id, mutual
        FROM userprofile_usercontact
        GROUP BY first_user_id) s 

    GROUP BY user_id ORDER BY sm DESC

Answer 3

Select sum(field) from (
  Select ....
  Union all 
  Select ....
 ) atable
where ....